1. Field of the Invention
This invention relates to the analysis of materials, and more particularly to a method and apparatus for analyzing selected properties of a ferromagnetic material by applying an alternating magnetic field to the material, and analyzing the resulting acoustic emissions induced in the material.
2. Description of the Prior Art
It is known that ferromagnetic materials will emit acoustic radiation when brought into an alternating magnetic field. This property has been used in various nondestructive testing techniques, such as the investigation and verification of structural integrity, and the detection and location of cracks and other structural flaws. A summary of the history and applications of acoustic emission work for both ferromagnetic and other materials is provided in Volume XI, Physical Acoustics Principles and Methods, edited by Warren P. Mason and R. N. Thurston, Academic Press, 1975, Chapter 6 (by Arthur E. Lord, Jr.).
Various properties of a ferromagnetic specimen will affect its acoustic emission response to an applied magnetic field. These properties include the stress to which the specimen is presently subjected or has been subjected in the past; strain or plastic deformation; the material's microstructure, which can be effected by various types of heat treatment; and chemical variations, such as differences in the carbon content of the specimen under investigation as compared to similar specimens. It has been suggested that acoustic emissions offer a method for measurement of at least some of these properties. For example, in a paper by Hideo Kusanagi, Hideo Kobayashi, and Hiroaki Sasaki originally published in Japanese and entitled "AE Characteristics and Stress Dependancy of Magnetization Process of Ferromagnetic Materials, " published in "The Proceedings of the First National Conference on Acoustic Emission," held December 1977 Tokyo, Japan, pages 157-162, it was suggested that the stress dependancy of magnetically induced acoustic emissions could be utilized as a stress measurement. The authors recognized, however, that an instrumentation problem is existed in the measurement of acoustic emission intensity because of the effects of varying contact conditions between the sample and the acoustic emission transducer.
In addition to this instrumentation problem, there are other limitations on the use of acoustic emissions for measuring the properties of ferromagnetic materials. First, there is a fairly large degree of uncertainty, perhaps in the order of ten percent, in comparing a measured level of acoustic emissions to what has previously been determined to be a standard response curve. In other words, because of variations in the testing environment and between samples, it cannot be said that a given level of induced acoustic emissions in a sample can be related to a standard curve relating acoustic emissions to stress for the sample material, to exactly determine the stress on the sample. Also, because a variety of factors other than stress can affect the acoustic emission response of a given sample, there is no certainty that, for a given value of acoustic emission generated in response to a particular level of magnetization, a unique level of stress or other property has been determined.
Furthermore, in certain cases when the acoustic emission response is plotted over a range of values for a given property such as stress, the same acoustic emission level can correspond to more than one stress level. Accordingly, there may be an ambiguity in the significance of the acoustic emission level. On the other hand, in some situations the level of acoustic emissions will vary only slightly or not at all over a significant portion of the range of property values. This makes the significance of the acoustic emission level somewhat vague in that area, since it might correspond to any one of the property values within such portion of the range.
There is accordingly a continuing need for a method and apparatus which can utilize the significant amount of information available in magnetically induced acoustic emissions from ferromagnetic materials, but which are more accurate and reliable than those presently available.